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E-HICH (Enhanced HARQ Information Channel)

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The Enhanced Hybrid Automatic Repeat Request Information Channel (E-HICH) is a feature in the 3rd Generation Partnership Project (3GPP) Long-Term Evolution (LTE) wireless communication standard that enables the transmission of retransmission requests from the User Equipment (UE) to the base station in a reliable and efficient manner. In this article, we will discuss E-HICH in detail, including its operation, features, and advantages.

Introduction to E-HICH

The E-HICH is a protocol used to enhance the Hybrid Automatic Repeat Request (HARQ) mechanism in LTE. HARQ is a communication protocol that enables error correction in wireless communication systems by retransmitting packets that are not correctly received at the receiver. HARQ improves system reliability and provides an efficient way of correcting errors that occur during the transmission of data packets. However, in the case of LTE, the existing HARQ mechanism was not effective in handling a large number of retransmission requests, which led to latency issues and degraded system performance.

To overcome this problem, the 3GPP standard introduced the E-HICH protocol in LTE Release 9. E-HICH provides a reliable and efficient way of transmitting retransmission requests from the UE to the base station, reducing the latency and enhancing the overall system performance.

E-HICH Operation

The E-HICH protocol is designed to work in conjunction with the HARQ mechanism. When the UE detects a transmission error, it sends a HARQ feedback message to the base station, indicating the need for retransmission of the lost packet. In LTE, the HARQ feedback message is sent on a dedicated control channel called the Physical Uplink Control Channel (PUCCH).

In the case of E-HICH, the retransmission request message is sent on a separate channel called the E-HICH channel. The E-HICH channel is a dedicated, low-latency channel that is used only for transmitting retransmission requests. The E-HICH channel is a unidirectional channel, which means that it is used only for transmitting messages from the UE to the base station and not in the other direction.

The E-HICH channel is a Physical Uplink Shared Channel (PUSCH), which means that it shares the same frequency band as the data channel used for transmitting user data. However, the E-HICH channel uses a different modulation scheme and coding rate than the data channel to ensure that the retransmission requests are transmitted reliably and efficiently.

The E-HICH message consists of two parts: the E-HICH header and the E-HICH payload. The E-HICH header contains information about the UE, including the UE identity, the number of retransmission requests, and the type of retransmission request. The E-HICH payload contains the actual retransmission request messages.

The E-HICH message is transmitted using a time division multiplexing (TDM) scheme, which means that the message is transmitted in separate time slots that are allocated to each UE. The time slots are allocated dynamically based on the number of UEs that require retransmission requests.

E-HICH Features

The E-HICH protocol provides several features that enhance the reliability and efficiency of the retransmission request mechanism. Some of the key features of E-HICH are:

  1. Low-latency: The E-HICH channel is designed to provide low-latency transmission of retransmission requests. This ensures that the retransmission requests are transmitted quickly and efficiently, reducing the time taken for retransmission and improving the overall system performance.
  2. Priority handling: The E-HICH channel is given priority over other channels in the system, ensuring that the retransmission requests are transmitted without delay. This helps in reducing latency and improving system performance.
  3. Multiple retransmission requests: The E-HICH protocol allows the UE to send multiple retransmission requests in a single message, reducing the overhead associated with transmitting multiple messages. This feature ensures that the retransmission requests are transmitted efficiently, improving system performance.
  4. Retransmission request types: The E-HICH protocol supports different types of retransmission requests, including Normal, Chase, and Multi-ACK, allowing the UE to request retransmission in different scenarios. Normal retransmission requests are used when the UE detects an error in the received packet, while Chase retransmission requests are used when the UE receives a new packet before receiving the retransmission of the previous packet. Multi-ACK retransmission requests are used when the UE receives multiple packets correctly and requests retransmission of a lost packet.
  5. E-HICH feedback: The base station provides feedback to the UE on the status of the retransmission request using the E-HICH feedback message. The E-HICH feedback message indicates whether the retransmission request was successful or not, allowing the UE to take appropriate action.

Advantages of E-HICH

The E-HICH protocol provides several advantages over the existing HARQ mechanism in LTE. Some of the key advantages of E-HICH are:

  1. Improved system performance: E-HICH reduces the latency associated with retransmission requests, improving system performance and providing a better user experience.
  2. Efficient retransmission: E-HICH allows the UE to send multiple retransmission requests in a single message, reducing the overhead associated with transmitting multiple messages. This feature ensures that the retransmission requests are transmitted efficiently, improving system performance.
  3. Low-latency transmission: E-HICH provides a dedicated channel for transmitting retransmission requests, reducing the latency associated with transmitting these requests. This ensures that the retransmission requests are transmitted quickly and efficiently, improving system performance.
  4. Flexible retransmission: E-HICH supports different types of retransmission requests, allowing the UE to request retransmission in different scenarios. This flexibility ensures that the retransmission requests are tailored to the specific needs of the UE, improving system performance.

Conclusion

In conclusion, the E-HICH protocol is a significant enhancement to the HARQ mechanism in LTE. E-HICH provides a reliable and efficient way of transmitting retransmission requests, reducing the latency associated with these requests and improving system performance. E-HICH offers several features that enhance the reliability and efficiency of the retransmission request mechanism, including low-latency transmission, priority handling, multiple retransmission requests, and flexible retransmission. The advantages of E-HICH make it an essential feature of the LTE standard, providing a better user experience and improving system performance.